The present invention is directed to a vertical geometry light emitting diode. More specifically, the present invention is related to a vertical geometry light emitting diode which emits ultraviolet, more preferably deep ultraviolet light.
Ultraviolet (UV) light emitting diodes (LEDs) are typically grown on electrically insulating substrates such as sapphire or bulk aluminum nitride. This requires the metallic contacts, often referred to as an anode and cathode, to be located on the same side of the final epitaxial wafer to allow current to pass through the diode. This geometry is typically referred to as a laterally conducting geometry since the electrodes are laterally displaced relative to each other. Non-exhaustive examples of lateral displaced geometry LEDs are provided in U.S. Pat. No. 7,202,506 and U.S. Pat. Appl. Publ. No. 2006/0091786 both of which are incorporated herein by reference.
A deficiency of LEDs with laterally conducting geometry is current crowding which results from forcing the current to travel laterally through a semiconductor with finite resistance prior to traveling vertically through the LED active region. Lateral current crowding increases proportional to the resistance of the material used for lateral conduction of the current. For UV emitting diodes the current crowding is particularly detrimental because as the wavelength of emission decreases the resistance of material used for the lateral conduction of current typically increases. While not limited to any theory, this is believed to be due to the increase in bandgap energy of the laterally conducting material to maintain transparency of the material relative to the wavelength of emission.
Deep ultraviolet (DUV) light-emitting diodes (LEDs) operating between 200 and 365 mm are expected to demonstrate great utility in many applications including as excitation sources in bioaerosol fluorescence detection systems. Therefore, there has been an ongoing desire to develop an LED structure, and method of producing the LED with improved quality and with increased manufacturing efficiency and reproducibility.
Vertical geometry LEDs mitigate many of the deficiencies associated with lateral geometry LEDs.
Vertical geometry LEDs and methods of manufacturing them are described in U.S. Pat. Nos. 7,001,824 and 7,148,520; U.S. Patent Application Publication Nos. 2006/0267043; 2007/0001190 and 2006/0071230 as well as in “Vertical Injection Thin-Film AlGaN/AlGaN Multiple-Quantum-Well Deep Ultraviolet Light-Emitting Diodes”, Zhou et al., Applied Physics Letters 89, 241113 (2006) and “Vertical Faceted Lateral Overgrowth of GaN on SiC with Conducting Buffer Layers Using Pulsed Metalorganic Chemical Vapor Depostion” Fareed et al., Applied Physics Letters vol. 77, number 15, 9 Oct. 2000, pages 2343-2345 all of which are incorporated herein by reference.
In general, vertical geometry LEDs are formed by sequential layering of functional layers upon a substrate. After formation of the functional layers the diode is removed from the substrate by laser assist wherein one or more of a series of buffer layers comprising AlxInyGa1-x-yN between the LED and substrate are melted or sublimated.
The present invention provides an improved buffer layer which facilitates removal of the LED from the substrate and an improved LED formed thereby.